Three dimensional printing of cell-laden hydrogels is of interest for tissue engineering due to the ability to create pre-seeded three dimensional (3D) structures with defined shape and internal geometry. However, hydrogel 3D printing is challenged by the fact that most hydrogel precursors are solutions that cannot form self-supporting structures. In addition, bioink development often presents a struggle between achieving printability and achieving biological compatibility. Conventional bioinks of hydrogel precursor solutions possess low viscosities and are conventionally cross-linked to form a gel either during or after the printing process. Post-printing cross-linking results in a printed layer lacking definition and resolution since solutions often diffuse rapidly after extrusion. Furthermore, crosslinking solutions post-printing is incompatible with printing multi-layer structures.
To overcome the poor structural definition of solution phase bioinks, researchers have attempted a few strategies including increasing polymer concentration, co-printing with a high shape fidelity support ink and layer-by-layer cross-linking; yet, these strategies have limitations. For example, relatively high polymer weight fractions (>5 wt %) can inhibit encapsulated cell spreading, migration, proliferation, and consequently tissue formation. In the case of co-printing with non-sacrificial support inks, the support ink may have mechanical properties ideal for maintaining shape and stability, but these may not be optimal for soft, non-load bearing tissues. The third strategy, layer-by-layer cross-linking, must occur extremely rapid to yield defined strands, yet too rapid cross-linking can result in nozzle clogging and poor inter-layer adhesion. Furthermore, co-printing multiple materials that utilize different solidification mechanisms, such as ionic, ultraviolet, chemical, or temperature-induced cross-linking, may not be feasible using layer-by-layer cross-linking.
Unlike solution phase bioinks, gel phase bioinks have been rarely explored. (See A. Skardal, J. Zhang, G. D. Prestwich, Biomaterials 2010, 31, 6173; D. L. Cohen. E. Malone. H. Lipson, L. J. Bonassar, Tissue Eng. 2006, 12, 1325; and A. Skardal, J. Zhang, L. McCoard, X. Xu, S. Oottamasathien, G. D. Prestwich, Tissue Eng. Part A 2010, 16, 2675.) Practical, efficient, functional, and multi-material bioprinting has yet to be reported with gel phase bioinks.